Insulator for an organic electronic component

ABSTRACT

The invention concerns an insulator for an organic electronic component, in particular, for an organic field-effect transistor (OFET) or for an organic capacitor. The insulating material is characterized in that it includes an almost constant relative dielectric constant, even in case of frequency variation in wide ranges, for example, between 1 Hz and 100 kHz.

The invention concerns an insulator for an organic electronic component,in particular, for an organic field-effect transistor (OFET) and/or foran organic capacitor.

We know from C. J. Dury et al., Applied Physics Letters 73 1998, p. 108,that polyhydroxystyrene (PHS) is used as an insulator in OFETs. The maindisadvantage of said material is that there is no known possibility thusfar to structure said insulator economically. Loose ions inside thematerial represent an additional problem, which lead to an extremely lowswitching behavior. Moreover, PHS is relatively expensive.

Commercially available photosensitive resist (SC100, Olin Hunt) was usedas an insulator in a more recent publication (G. H. Gelinck et al.,Applied Physics Letter 77 2000, p. 1,487). Due to the structuring of thephotosensitive resist, the layers beneath suffer major corrosion ordestruction, which is a substantial disadvantage of this method. Thismakes it practically impossible to use said insulator on alreadyexisting semiconductor layers such as, for example, polyalkylthiophene.However, the insulating layer above the semiconducting layer isdeposited to produce the OFET, in which the source and/or drainelectrodes are embedded. Damage to the already existing semiconductinglayer cannot be tolerated during the manufacturing process.

Polyimide was also presented as insulating material (J. A. Rogers etal., IEEE Electron Devices Letters, Volume 21, Number 3, 2000, p. 100).Even when using said material, there is the fear of causing damage tothe already finished OFET layers, since said material can only beprocessed at extremely high temperatures (˜400° C.). Since organicsemiconductors or conductors can typically survive only significantlylower temperatures without damage (less than 200° C.), polyimide cannotbe used in fully organic OFETs.

Independent of the processing characteristics of the familiar materials,an insulator, whose dielectric constant remained basically constant whenthe emitted frequency is changed, could not be found thus far. Rather,all these materials demonstrate a frequency-dependent change in thedielectric constant, which affects entire ranges.

Therefore, it is the challenge of the present invention to provide aninsulator for a field-effect transistor, which at least partiallyconsists of organic material and which overcomes the disadvantages ofprior art.

The subject matter of the invention concerns an insulator for an organicelectronic component, in particular, for an organic field-effecttransistor and/or a capacitor, which is at least partially based onorganic material, where the dielectric constant of the insulating layeressentially remains constant in a frequency range between 1 Hz and 100kHz.

According to one embodiment, the insulator is comprised ofpolyisobutylene or uncrosslinked EPDM (Ethylene Propylene Diene Monomer)as base polymer (main component), which are only soluble in nonpolarhydrocarbons (hexane, heptane). The homogenous thickness of the layerthat can be achieved with the material is between approx. 2 μm and 250μm, whereby these layers still possess sufficiently high insulationcharacteristics. Said material can be structured very easily, thuspermitting hole contacts, which is another major advantage of saidmaterial (e.g. by means of lithography).

According to another embodiment, the insulating material is comprised ofa commercially available PVDC-PAN-PMMA copolymer with the generalformula(—CH₂Cl₂—)_(x)—(—CH₂CH(CN)—)_(y)—(—CH₂C(CH₃) (CO₂CH₃)—)_(z),wherein x, y, z, in each case, independently from one another, mayassume values between 0 and 1, preferably values as indicated in theexamples.

The PVDC PAN PMMA copolymer is preferably used in combination with HMMM(hexamethoxy methal melamine) and/or Cymel crosslink components, whoseratio can be varied widely (dissolved in dioxane). Said material alsopermits very simple structuring without being crosslinked yet at thesame time. Said material can be crosslinked at very low temperatures(approx. 70° C.) and becomes then resistant to all subsequent steps thatare necessary to complete an OFET and to put together an integratedcircuit.

According to one embodiment, the insulating compound comprised of a basepolymer with the general formula[A_(x)/B_(1-z)],is used,wherein A, for example, is polyhydroxystyrene and B ispoly(styrene-co-allyl-alcohol) such as, for example, polyvinyltoluol,poly(alpha-methylstyrene).

In particular, compounds such as, for example, [50%polyhydroxystyrene/50% poly(styrene-co-allyl-alcohol)], dissolved inpolar solvents such as, for example, dioxane, are preferred in thisregard. A major advantage of said material is that a layer can bedeposited on P3AT with very little damage.

Finally, according to another embodiment, an insulator is usedcomprising a compound of two copolymers, with the general formula[A_(z)/B_(y)],wherein, in particular, a compound ofpoly(vinyltoluene-co-alpha-methylstyrene)/poly(styrene-co-allyl-alcohol)is suitable. The x and y indices may thereby be equal or unequal andassume values between 0.5 and 1. There is a particular preference for xand y to be equal. Again, the compound is preferably dissolved in polarsolvents, in particular, dioxane.

Surprisingly, the materials mentioned fulfill characteristic profilesallowing their use, in particular, as insulating layer in OFETs:

This is particularly so, since an insulating layer made up of one or acompound of several mentioned materials fulfills the following process,electrical and mechanical requirements and, at the same time, is a veryinexpensive material system:

-   a) Process requirements:-   b)    -   The insulating layer easily dissolves in conventional organic        solvents such as, for example, dioxane, butanol and other        alcohols, etc.    -   Depositing the insulating layer onto already existing OFET        layers (e.g. semiconductor layer) does not damage these layers,        either through corrosion or etching, nor does it change their        characteristics.    -   After depositing, the insulating layer can be structured. Also,        structuring does not negatively affect existing layers.        Structuring is absolutely necessary in order to create        integrated circuits, which consist of several OFETs, since        structuring is required to make link circuits between the gate        electrode of one OFET and the source or drain electrode of        another OFET possible.    -   After structuring, the insulating layer is chemically and        thermally stabile vis-à-vis the process steps that are required        to deposit and structure subsequent OFET layers (e.g. gate        electrode).-   b) Electrical requirements:    -   The relative dielectric constant of the insulating layer is        nearly constant in a frequency range between 1 Hz and 100 kHz.        The relative dielectric constant is considered “nearly constant”        in this context, if it varies by 50% or less.    -   Preferably, the relative dielectric constant of the insulating        layer has at least a value of about 2 for the mentioned systems,        thus allowing OFETs to be produced that work at low voltage.    -   It is advantageous that leakage currents through the insulating        layer, even in the case of very thin layers, are negligibly        small vis-à-vis the source-drain current, i.e. they preferably        lie below 1 nA (depending on the OFET geometry).    -   The dielectric strength of the insulating layer is high and has        a preferred value of at least 5*10⁵ V/cm.    -   Preferably, the insulating material should not contain any        movable impurities (e.g. ions).    -   Preferably, the threshold voltage of the OFET is not displaced        by the insulating system.    -   Mechanical requirements:    -   To a certain extent, the insulating layer is resistant against        mechanical force such as bending, stretching or compressing.    -   Depositing the insulating layer by spin-coating, doctoring,        printing or spraying is done in such a way as to create a        plane-parallel, even, homogeneous layer free of defects.

To produce a complete OFET, structurable layers of either photosensitiveresist or metal are deposited on the insulating layer. Afterstructuring, the insulating layer can be precisely removed with suitablesolvents and thus structured as well. This way, the insulating layer isalways structured at temperatures below 100° C., so that processing inthis way has no negative effect on already existing functional layers(e.g. semiconductors).

The excellent electrical characteristics, i.e., high dielectricalconstant, high breakdown voltage and low leakage currents of thematerial systems under consideration continue to permit the productionof relatively thin insulating layers, which leads to a drastic reductionof the required gate voltage to preferred values below 10 V.

In this context, the term “organic material” or “organic functionalpolymer” comprises all kinds of organic, metal-organic and/ororganic-inorganic synthetic materials (hybrids), particularly those,which are referred to, for example, in the English language, as“plastics.” This concerns all types of materials with the exception ofsemiconductors, which form classic diodes (germanium, silicon), and thetypical metallic conductors. Thus, dogmatically speaking, there are noplans of limiting the use to organic, that is, carbon-containingmaterials, rather, the wide use of, for example, silicon is alsoconsidered. Moreover, the term should not be subject to any limitationswith regard to molecular size, in particular, limitations to polymerand/or oligomer materials, rather the use of “small molecules” is alsoquite possible. The word component “polymer” within functional polymeris historic and insofar contains no information about the existence ofactual polymer compounds.

Following, the invention will be explained on the basis of someexamples, which describe embodiments of the invention:

EXAMPLE 1 Use of Polyisobutylene (PIB) as an Insulator

-   -   0.4 g of PIB (Aldrich) was dissolved in 9.6 g of hexane at room        temperature;    -   the solution was filtered through a PTFE 0.45 μm syringe filter;    -   the solution was then spin-coated onto the substrate (4,000 rpm        for 20 seconds), which was already fit with source/drain        electrodes and semiconductors (top-gate design). The result was        a very homogenous layer, approx. 260 nm thick    -   the sample was dried under a dynamic vacuum for approx. 30        minutes at room temperature    -   then a thick layer of photosensitive resist was deposited on the        insulator, exposed and developed under normal circumstances;    -   the sample was immersed in a hexane bath and the insulator was        stripped in the areas that were free of resist    -   the remaining resist was removed with a suitable solvent

EXAMPLE 2 Use of PVDC-PAN-PMMA (x=0.89, y=0.03, z=0.08) as Insulator

-   -   0.4 g of PVDC-co-PAN-co-PMMA (Aldrich) was dissolved in 9 g of        dioxane at 40-50° C.    -   0.5 g of cymel 327 (Cytec Industries, Inc.) and 0.1 g of campher        sulfonic acid were then added and stirred for a few seconds;    -   the solution was filtered through a PTFE 0.45 μm syringe filter;    -   the solution was spin-coated onto the substrate (8,000 rpm for        20 seconds), which was already fit with source/drain electrodes        and semiconductors (top-gate design). The result was a very        homogenous layer, approx. 400 nm thick    -   the sample was dried under a dynamic vacuum for approx. 30        minutes at room temperature;    -   the layer was vacuum-coated with a thin gold layer, which in        turn was structured by means of photolithography (photosensitive        resistant, then etched with a potassium carbonate solution)    -   said deposited metal mask allows structuring of the insulating        layer in that the now freed insulating surfaces can be removed        with a toluene-soaked rag    -   the remaining gold residue was removed with a potassium        carbonate solution    -   the last step was to crosslink the insulator (10 minutes at 90°        C.)

EXAMPLE 3 Use of [50% polyhydroxystyrene/50%poly(styrene-co-allyl-alcohol) as Insulator

Said polymer compound was then dissolved with dioxane and filtered witha 0.2 μm filter. Then, the insulating layer was then pre-baked on thehot plate at 100° C. for 30 minutes. As in example 2, structuring isalso carried out by means of “metal masks.”

The insulating material according to the invention shows no substantialfrequency-dependent variation of the dielectric constant. Either theorientation of existing anisotropic molecules or a lack of mobile chargecarriers as well as mobile ions may be responsible for this phenomenon.

At any rate, no significant variation of the dielectric constant,exceeding approx. 50%, could be established within a frequency range ofalmost 100 kHz.

1. An insulator for an organic electronic component comprising: a layer of material which is at least partially based on organic material, wherein the dielectric constant of the insulating layer of material remains substantially constant within a frequency range between 1 Hz and 100 Hz; the layer of material being arranged to be dimensioned including a thickness and outer peripheral dimensions to form one of an organic field-effect transistor and capacitor.
 2. An insulator in accordance with claim 1, comprising one of polyisobutylene and uncrosslinked EPDM (Ethylene Propylene Diene Monomer) as a base polymer.
 3. An insulator in accordance with claim 1, comprising as base polymer a commercially available PVDC-PAN-PMMA copolymer with the general formula (—CH₂Cl₂—)_(x)—(—CH₂CH(CN)—)_(y)—(CH₂C(CH₃)(CO₂CH₃)—)_(z), wherein x, y, z, in each case, independently from one another, may assume values between 0 and
 1. 4. An insulator in accordance with claim 1, comprising a base polymer with general formula [A_(x)/B₁-z], wherein A, for example, is polyhydroxystyrene and B is poly(styrene-co-alyl-alcohol), polyvinylalcohol and/or poly-alpha-methylstyrene.
 5. An insulator in accordance with claim 4, wherein the base polymer is a compound comprising 50% polyhydroxystyrene and 50% poly(styrene-co-allyl-alcohol).
 6. An insulator in accordance with claim 1, comprising as base polymer a compound of two polymers, with the general formula [A_(z)/B_(y)], with A equals poly(vinyltoluene-co-alpha-methylstyrene) and B equals poly(styrene-co-allyl-alcohol), wherein the values of x and y may be equal or unequal and assume values between 0.5 and
 1. 7. An insulator in accordance with claim 6, wherein the values of x and y are equal.
 8. An insulator, in accordance with one of the claims 3 through 7 wherein the base polymer is dissolved in one of a polar solvent and a polar mixture comprising at least two solvents.
 9. The insulating layer of claim 1 including a circuit formed on a surface of said layer.
 10. The insulating layer of claim 1 including an OFET circuit formed on a surface of said layer.
 11. The insulating layer of claim 1 including a transistor circuit formed on a surface of said layer. 